An impact ionization and transit time (IMPATT) diode is a two terminal semiconductor negative conductance microwave device which operates by a combination of avalanche multiplication and transit time effects. Generally, an IMPATT diode has a simple pn junction structure which is reverse biased to avalanche breakdown. An AC voltage is then superimposed on the DC bias. The carriers generated by the avalanche process are swept through the drift region of the IMPATT diode to the terminals of the device. The AC component of the resulting current can be approximately 180.degree. (.pi. radians) out of phase with the applied voltage under proper conditions of bias and device configuration, thereby giving rise to negative conductance and oscillation in a resonant circuit. IMPATT devices can convert DC to microwave AC signals with high efficiency and are very useful in the generation of microwave power for many applications.
The classical device configuration for the IMPATT diode has been originally set forth by W. T. Read. Although IMPATT operation can be obtained in simpler structures, the Read diode is an exemplary device for illustration of the basic principles.
With particular reference to FIG. 1 (prior art), a Read diode 10 includes a first device layer 12, an avalanche layer 14, a drift layer 16 and a first contact layer 18. The first device layer 12 is formed from p-type Gallium Arsenide (GaAs) and the avalanche layer 14 is formed from n-type GaAs. The first layer 12 and the avalanche layer 14 thus form a pn junction 20. The drift layer 16 is n-type GaAs (although essentially intrinsic GaAs may also be used) and the contact layer 18 is highly doped n-type GaAs. Additionally, a second contact layer of highly doped p-type GaAs may be provided adjacent the first layer 12.
Although the Read diode 10 is exemplarily described as being fabricated from GaAs, any suitable semiconductor material may be used. It is to be noted that each layer of the prior art Read diode 10 is fabricated from the same semiconductor material. It is also within the ordinary skill of the art to construct the Read diode 10 in a n-p-i-p configuration in which holes resulting from avalanche multiplication drift through the drift layer 16. For either polarity configuration of the Read diode 10, under reverse bias of the pn junction 20, avalanche multiplication occurs in the avalanche region from minority carriers thermally generated in the first device layer 12, as well known in the art. The avalanche generated majority carriers in the avalanche region 14 then drift through the drift layer 16 towards the first contact layer 18.
Although detailed calculations of the operation of the Read diode 10 are complicated and generally require computer solutions, the basic physical mechanism is simple. Essentially, the Read diode 10 operates in a negative conductance mode when the AC component of current is negative over a portion of the cycle during which the AC voltage is positive, and vice versa. The negative conductance occurs because of two processes, causing the current to lag behind the voltage in time:
(1) A delay due to the avalanche process; and PA0 (2) a further delay due to the transit time of the carriers across the drift layer 16.
If the sum of these delay times is approximately one half cycle of the operating frequency, negative conductance occurs and the Read diode 10 can be used as an oscillator, a locked oscillator, or as an amplifier.
More particularly, the AC conductance is negative if the AC component of carrier flow drifts opposite to the influence of the AC electric field. For example, with the DC reverse bias at the pn junction 20 by appropriate application of a voltage to the first contact layer 18 and second contact layer 22, electrons drift from the avalanche layer 14 towards first contact layer 18 through the drift layer 16, under the influence of the resulting DC electric field through the drift layer 16 as expected. If an AC voltage is superimposed such that the magnitude of the electric field decreases during the positive half cycle of the superimposed AC voltage (decreasing the magnitude of the reverse bias), the drift of electrons through the drift layer 16 does not exhibit a corresponding decrease. Instead, in IMPATT operation the drift of electrons through the drift layer 16 actually increases while the magnitude of the AC field is decreasing.
In describing this aspect of the IMPATT operation, it may be assumed that the avalanche layer 14 is very narrow and that all avalanche multiplication takes place in a thin region near the pn junction 20. As is known in the art, the integral of the field times the ionization coefficient must be at least unity for the avalanche process to begin. If the DC reverse bias is such that a critical electric field E.sub.a for avalanche conditions is just met in the pn space charge region, avalanche multiplication begins (at t=0). Holes generated in the avalanche layer 14 move to the p-type first device layer 12 and second contact layer 22 and electrons enter the drift layer 16. The Read diode 10 is then mounted in a resonant microwave circuit so that an AC signal v(t)=-vsin(.omega.t) can be maintained at a given frequency. As the applied AC voltage goes negative (enhancing the magnitude of the reverse bias) at .omega.t=0.sup.+, more and more electrons are generated in the avalanche region. In fact, the pulse of electrons generated by the multiplication process continues to grow as long as the electric field has an absolute value of intensity greater than E.sub.a. It can be shown that the partial current due to avalanche increases exponentially with time while the magnitude of the field is above the critical value. The important result of this growth is that the electron pulse reaches its peak value not at .omega.t=.pi./2 when the AC voltage is most negative but at .omega.t=.pi.. Therefore, there is a phase delay of .pi./2 inherent in the avalanche process itself. The further delay is provided by the drift layer 16. Once the avalanche multiplication stops (.pi..ltoreq..omega.t&lt;2.pi.), the pulse of electrons simply drifts towards the n-type first contact 18. But during this period, the AC voltage is positive. Therefore, the dynamic conductance is negative, and energy is supplied to the AC field.
If the length of the drift layer 16 is chosen properly, the pulse of electrons is collected at the n-type first contact layer 18 just as the voltage cycle is completed (t=2.pi.), and the cycle then repeats itself. The electron pulse will drift through the length L of the drift layer 16 during the positive half cycle of the AC voltage (.pi..ltoreq..omega.t&lt;2.pi.) if the transit time T.sub.t is chosen to be one half of the oscillation period; or EQU T.sub.t =L/v.sub.d =1/2f or f=v.sub.d /2L=1/2T.sub.t
where f is the operating frequency and v.sub.d is the drift velocity for electrons. Therefore, in the Read diode 10, the optimum frequency is one half of the inverse transit time of electrons across the drift region v.sub.d /L. In choosing an appropriate resonant circuit for the IMPATT diode 10, the parameter L is critical. To maximize efficiency and minimize harmonic generation, it is desirable that the pulse remains localized during its transit. GaAs and Indium-Phosphide (InP) possess diffusion coefficient versus electric field properties that maintain this pulse localization.
For example, taking v.sub.d =10.sup.7 cm/sec for Silicon, the optimum operating frequency for a device with a drift layer length of five microns is f=10.sup.7 /2(5.times.10.sup.-4)=10.sup.10 Hz, or 10 GHz. Negative resistance is exhibited by the diode 10 for frequencies somewhat above and below this optimum frequency for an exact 180.degree. phase delay. Careful analysis of the small single impedance model shows that the minimum frequency for negative conductance varies as the square root of the DC bias current for frequencies in the neighborhood of that described above. It is to be understood that the above description of the operation of the prior art Read diode 10 is an ideal first order analysis.
In general, the prime performance criteria for IMPATT diodes is the conversion efficiency which is usually a percentage of the fraction of the direct input power into the diode that is converted to usable microwave energy. Since a typical IMPATT diode is physically small, typically 5-20 thousandths of an inch in diameter, the amount of input power is limited by the generation of excessive thermal energy. As the IMPATT diode becomes more efficient in the conversion of input power to usable microwave energy, the more microwave energy is available for a given input specified power.
Theoretical models of IMPATT diodes indicate that efficiency is primarily a function of two considerations. The first consideration is that the voltage drop across the avalanche layer must be held to a minimum compared to the voltage drop across the drift layer. This can be achieved by using a very narrow avalanche layer, or by reducing the electric field through the avalanche layer. The field must, however, still be maintained at or above the minimum field strength E.sub.a required to sustain impact ionization. There are also circuit related factors which bear upon the electric field strength. For example, as the frequency of operation is increased, the capacitance of the IMPATT diode must be reduced. This capacitance is a function of the diode's breakdown voltage, depletion layer capacitance and doping levels.
The second consideration is that the high efficiency IMPATT diode operates in a "premature collection mode." A full discussion of the premature collection mode is outside the scope of the present disclosure. Numerous references on IMPATT diodes which quantitatively analyze this mode may be consulted. Qualitatively, the premature collection mode results from time varying change of the effective length of the drift layer. The effective length of the drift region is defined by the extend of it which is depleted of majority carriers, the "depletion zone.[ It is in this depletion zone that the remaining electric field causes the generated carriers to drift at their maximum saturation velocities. This effective length changes as the depleted region varies in length in response to the AC voltage. For large signal AC voltages, the depletion region extends substantially through the drift layer for one-half cycle. The premature collection modes requires a relatively low doped drift layer as compared to the avalanche layer. Again, this requirement places limits on the allowable doping levels. These constraints place effective limits on the maximum conversion efficiency. For the single drift Read-type profile, such as the Read diode 10 described hereinabove, typical values that have been achieved in commercially available devices are 22-24% for X-band devices (8-12 GHz), and 19-21% for K-band devices (18-22 Ghz).